1. Field of the Invention
The present invention relates to a laser diode driving circuit for an optical device and a laser diode controlling apparatus including the laser diode driving circuit. More particularly, the present invention relates to a laser diode driving circuit that protects a laser diode by preventing an excessive amount of laser diode driving current from being input to the laser diode and a laser diode controlling apparatus including the laser diode driving circuit.
2. Description of the Related Art
In general, no-touch optical apparatuses, such as CD and DVD readers and players, project light onto a disk by using a laser diode. The reflected light from the disk is converted into an electrical signal by using a light receiving diode, and the data from the disk is read by processing the electrical signal. The intensity of light incident on the light receiving diode needs to be maintained at a predetermined level so that the light reflected from the disk can be efficiently processed. For this, the power of the laser diode should also be maintained at a predetermined level, and this is why such no-touch optical apparatuses need a laser diode controlling apparatus.
FIG. 1 is a circuit diagram of a conventional laser diode controlling apparatus for controlling the power of a laser diode. Referring to FIG. 1, the conventional laser diode controlling apparatus includes an automatic laser power control (ALPC) circuit 110, which receives an output signal of a photodiode 101. The ALPC circuit 110 sets a reference voltage based on a variation in the intensity of the output signal of the photodiode 101, and outputs the reference voltage so that a stable optical power can be obtained. A laser diode driving circuit 120 outputs a current (hereinafter, referred to as laser diode driving current) for driving a laser diode 102 based on the reference voltage output from the ALPC circuit 110. A laser diode protecting circuit 130, which is installed between the laser diode driving circuit 120 and the laser diode 102, protects the laser diode 102 from static electricity or surge voltage possibly generated when installing a pickup unit 100 into or separating the pickup unit 100 from the conventional laser diode controlling apparatus. The pickup unit 100 preferably comprises the laser diode 102 and a photodiode 101.
In FIG. 1, a total of five nodes, that is, first through fifth nodes 1 through 5, are provided for the laser diode driving circuit 120. A bias point resistor R1 is disposed between the first and second nodes 1 and 2. In the laser diode driving circuit 120, a transistor Q constitutes an emitter follower amplifier. The base of the transistor Q is connected to the second node 2, the emitter of the transistor Q is connected to the fourth node 4, and the collector of the transistor Q is connected to the fifth node 5. A capacitor C1, which removes noise components, is disposed between the third and fourth nodes 3 and 4. The third node 3 is connected to a power supply voltage Vcc.
In a case where static electricity or surge voltage is generated, such as, when installing the pickup unit 100 into or separating the pickup unit from the conventional laser diode controlling apparatus, damage to the laser diode 102 can be prevented by charging or discharging the capacitor C2 and limiting the breakover voltage of a diode D1.
When the optical device operates, the base and emitter of the transistor Q preferably have predetermined voltages Vb and Ve, respectively. The voltages Vb and Ve are used as follows: Ve−Vb is approximately equal to 0.7 V, and (Vcc−Ve)/R2 is approximately equal to {Vcc−(0.7 V+Vb)}/R2 (where (Vcc−Ve)/R2 denotes current at the emitter of the transistor Q). The current at the base of the transistor Q is very low as compared to the current at the emitter of the transistor Q. Thus, the current at the collector of the transistor Q, which is the laser diode driving current, is substantially the same as the current at the emitter of the transistor Q.
However, if during the operation of the optical device, the base voltage of the transistor Q is lower than the target voltage level, the actual base voltage of the transistor Q is equal to Vb−Vs and the emitter current of the transistor Q is equal to {Vcc−(0.7 V+Vb−Vs)}/R2. The actual base voltage of the transistor Q is the voltage Vb′ at the second node 2. Also, the target voltage may be lower that the target voltage level by as much as Vs due to static electricity, surge voltages, or other factors. Therefore, a current more than {Vcc−(0.7 V+Vb)}/R2 by Vs/R2 is supplied to the laser diode 102 as a laser diode driving current. The laser diode 102 may be damaged if the laser diode driving current is beyond the rated tolerance of the laser diode 102. The laser diode protecting circuit 130, which is disposed between the pickup unit 100 and the laser diode driving circuit 120, can protect, to some extent, the laser diode 102 from static electricity or a surge voltage that may be generated when installing the pickup unit 100 into or separating the pickup unit 100 from the conventional laser diode controlling apparatus. However, the laser diode protecting circuit 130 is still incapable of fully protecting the laser diode 102 from an excessive amount of driving current generated due to a decrease in the voltage of the base of the transistor Q during the operation of the optical device.